High-Level Design

the client will send the requests to a load balancer, this one is needed to handle multiple requests as you mentioned there will be millions a day.

after this, the load balancer will forward the request to one of the servers, multiple servers are needed to handle a large amount of requests and even geo based in order to decrease the latency as much as possible.

the request reaches API Gateway, which either declines the call due to exceeding rate limit, or call the write service, in this case called shortener service, or the lookup one which is readonly. we separated these 2 servers to decouple write and reads as much as possible. shortener service then looks up in cache, for example Redis which will invalidate the cache every hour, if the request wasnt already computed, if so it returns it, if not it saves it in db and then caches it. in order to avoid collisions, a hash function can be used, if a collision does appear than we could add an extra predefined string to the original url and reapply the hash function.

the lookup service will use a copy of the master db, or even more copies to reduce latency, and it will fetch the long url from there and then redirect the user to it

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